The potentials are not directly observed. The potential parameters \(\theta \)  are
inferred through the observed interactions of pedestrian paths. Every loss function
evaluation is a simulation of all pedestrian paths in the given scenes.
Automatic differentiation is used to obtain the gradients of the potentials.
It is the same technique that is used to obtain the gradients for the parameter
update with Stochastic Gradient Descent (SGD) [1]}.
